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ISSN: 2321-8819 (Online) Asian Journal of 2348-7186 (Print) Impact Factor: 0.92 Multidisciplinary Studies Volume 3, Issue 2, Feb. 2015

Geochemistry of Mesoproterozoic metadolerite dykes and sills of basin, Meghalaya, NE India

K.P. Sarma*, J.J. Laskar, Niva Rani Devi, Nandita Mazumdar,

J. Mallikharjuna Rao and M. Venkateshwarlu 1Professor – (Rtd.), Department of Geological Sciences, Gauhati University, Guwahati – 781014, India. 2Assistant Professor, Department of Geological Sciences, Gauhati University, Guwahati – 781014, India. 3Assistant Professor, Arya Vidyapeeth College, Guwahati 781016, India. 4Research Scholar, Department of Geological Sciences, Gauhati University, Guwahati – 781014, India. 5Retired Scientist, National Geophysical Research Institute, Uppal Road, Hyderabad 500 606, India 6Scientist E, National Geophysical Research Institute, Uppal Road, Hyderabad 500 606, India. Abstract The Shillong Basin is the only Proterozoic (Purana) basin of NE India and is an intracratonic sag basin in nature. The basin is dominantly exposed in Meghalaya and partly in Karbi Anglong district of Assam and confined in the central and eastern parts of the Shillong plateau. This Proterozoic basin was developed during early Mesoproterozoic time. The metasedimentary rock associations of this basin are classified under Lower Metapelitic and Upper Quartzitic Formations closely associated with mafic magmatism in the form of mafic sills and dykes (metadolerite, locally named as Khasi greenstone). They are metamorphosed under greenschist – amphibolite transitional facies. The present study aims at generating geochemical characteristics of the metadolerites of the basin. Mafic dykes and sills of the basement of the Shillong plateau is of Cretaceous age and excluded from the present study. Geochemically, metadolerites are subalkaline and tholeiitic in nature and emplaced in continental setting. Geochemical data, mantle normalized spidergram and chondrite normalized REE patterns suggest that they are genetically related and probably derived from the same parental source magma undergoing shallow level fractionation. Keywords: Shillong Plateau, Shillong basin, Dolerite dykes and sills, Geochemistry.

Introduction evidences for distinct episodes of crust generation. Mafic dykes and sills occur in a wide variety of The chemical composition of mafic dykes and sills geological tectonic settings and their study provides provides clues to their crustal emplacement insights into geological events and continental conditions that maybe related to the prevalent breakups which ultimately help in reconstruction of tectonic stress regimes. The Shillong plateau has continents. Petrology and geochemistry of mafic played a vital role in the Precambrian history of the magmatic bodies document origin, emplacement and Indian plate and in particular northeast Indian region. growth of continental crust and provide vital The magmatic history and mafic emplacements of

Fig.1 Generalised geological map of Shillong basin showing the location of metadolerites

Available online at www.ajms.co.in 37 Geochemistry of Mesoproterozoic metadolerite dykes and sills of Shillong basin, Meghalaya, NE India.

the Shillong plateau are important as the plateau al., 1994). Yin et al. (2010) have established three forms the northeastern margin of Indian Peninsula episodes of igneous activity at 1600 Ma, 1100 Ma and records evidence of the evolving plate boundary. and 500 Ma witnessed in the rocks of the Shillong The mafic dykes and sills in Shillong basin, an plateau. The first episode of igneous activity at 1600 intracratonic rift related sag basin of Ma is related to the collision of two proto India Mesoproterozoic age, are metadolerites (named as continents and the ductile phase that took place at Khasi greenstone by Medlicott, 1869). They are 1100 Ma under contractional mechanism induced by metamorphosed along with arenaceous and assembly of Rodinia and Eastern Gondwana. This argillaceous sediments under greenschist – finding is correlatable with the findings of Chatterjee amphibolite transitional facies (Turner, 1968). These et al. (2006) who have presented monazite date from dykes were not given much attention as compared to Goalpara and Sonapahar area as 1596±15 Ma and other mafic dykes of Indian subcontinent. Some 570± 14 Ma. Bidyananda and Deomurari (2007) sporadic works on geochemistry and have given 1.5 to 2.6 Ga dates from basement palaeomagnetism have been done by Hazara et al. gneisses. However, Chatterjee et al. (op. cit) have (2008), Mallikharjuna Rao et al. (2009), Ray et al. arguably claimed that there is no Archean rock in the (2013), Sarma et al. (2014). But the present study Shillong plateau. aims at generating geochemical data, petrographic The lithoassociations of the Shillong basin information and interpretation of origin of these constituting Shillong group (earlier Shillong series of Proterozoic Metadolerite dykes and sills of the Medlicott, 1869), is classified under two formations, Shillong Basin which is an intracratonic Purana basin namely Lower Metapelitic Formation (LMF) and of the Shillong plateau and to demonstrate important Upper Quartzitic Formation (UQF). The UQF was features of their emplacement history and source named as “Mawphlang Formation” or Shillong material. Formation (Ahmed, 1981; Barooah and Goswami, Regional geological setting 1972), whereas LMF is known as Manai Formation Shillong plateau represents a detached part by Bhattacharjee and Rahman, (1985) or Tyrsad of the Indian sub continental craton. The E-W Formation by Barooah and Goswami (1972), trending Shillong plateau is an elevated block about Barapani Formation by (Ahmed, 1981). The LMF are 600-800m above Bangladesh plain and marked by quartz – mica schist, quartz– sericite geomorphologically appears as an inverted spoon schist, biotite- sericite schist and biotite schist. The shaped structure (Duarah and Phukon, 2011). The UQF representing the upper sequence of the Shillong uplift is related to collision of Indian and Tibetan basin are of massive to more micaceous types plates during Cenozoic time (Johnson and Alam, showing primary cross stratifications and ripple 1991; Nandi, 2000 and Bilham and England, 2001). marks. They are often intercalated with thinly The plateau is bounded to the north by Brahamaputra bedded metapelites and at the contact of lineament, to the south by Dauki fault, Indo Neoproterozoic granite, contact metamorphic Myanmar Mobile Belt (IMMB) to the east and rocks (hornfelse) are observed with well developed Dhubri – Yamuna lineament to the west and is andalusite crystals. tectonically separated by Garo-Rajmahal graben Metadolerites (Khasi greenstone of from Peninsular India (Eremenco et al., 1969). It Medlicott, 1869) occur as green coloured dykes, sills constitutes the leading edge of the Indian plate or isolated linear to curvilinear bodies.They often during its oblique collision with the Australo- occur as angular rafts, fragments and subrounded Antarctic plate at ca. 500-650 Ma (Chatterjee et al., xenolithic bodies within the younger granites. They 2006). The plateau is criss crossed by numerous are compact, medium grained and rarely coarse lineaments in NE-SW, N-S and EW sdirections grained and it exhibits spheroidal blocks due to (Chattopdhyaya and Hashmi, 1984). Further, a NW- weathering. Two types of mafic bodies – massive SE tectonic Kopili lineament dissects the plateau into and foliated, from type area Leitkroh in East Khasi two blocks namely Meghalaya massif and Mikir Hills district are found (Hazara et al., 2008). Both the massif (renamed as Karbi Anglong massif by Sarma types are essentially enclosed within Upper and Dey, 1996). Quartzitic Formation of Shillong Group. But a The Shillong plateau portrays a classic notable factor is observed that the foliated type is example of basement – cover relationship where essentially enclosed within the lower metapelitic Shillong Group (SG) acts as cover and basement by formation and massive type is confined within upper Basement Gneissic Group (BGG). The BGG quartzitic formation, the latter is marginally altered comprises of metamorphites including and affected by low grade metamorphism, central quartzofeldspathic gneisses, migmatites, part being still massive. amphibolites, calc-granulites, calc-silicate rocks, The Shillong Group of metasediments has high grade schist, basic and acid intrusives been dated as 1530 – 1550 Ma (Mitra, 1998) and 1.4 andpegmatites. They are metamorphosed under to 1.9 Ga (zircon dates of metasedimentary granulite - amphibolite transitional facies. The quartzites by Bidyananda and Deomurari, 2007). granite gneiss of BGG from Patharkhang area of Neoproterozoic granites were intruded into the West district of Meghalaya, is dated as Shillong basin (e.g. Mylliem pluton, Kyrdem pluton Paleoproterozoic age (Hazara et al., 2008; Ghosh et and South Khasi pluton), which have been dated as

Asian Journal of Multidisciplinary Studies, 3(2) February, 2015 38 Geochemistry of Mesoproterozoic metadolerite dykes and sills of Shillong basin, Meghalaya, NE India.

607±10 Ma (Crawford, 1969); 885 – 80 Ma (Ghosh appearance clearly indicate that there are two types et al., 1991, 1994 and Bhattacharyya and Ray of mafic bodies such as foliated and massive types Barman, 1998) andthey form another Formation and it is supported by petrographic features. The named as “Mylliem Formation” (GSI, 1972). massive type shows ophitic textures without Recently Sarma (2014) has attempted alteration effects (Fig.2A) and partly altered type reclassification of Shillong Group and their shows alteration and recrystallization of plagioclase underlying basement gneisses and proposed and clinopyroxene with relict ophitic texture (Fig. Shillong Supergroup in the light of lithostratigraphic 2B). Plagioclases are bytwonite (An71) in nomenclature. Conglomerates of the Shillong Group composition and show compositional zoning and separating underlying BGG and overlying Mesozoic effects of clouding in the central part while outer rim – Tertiary sequences are delineated in detail by Devi is more sodic which indicate that they are partially and Sarma (2006, 2010) and their strain history is affected bu low grade metamorphism. The established. Besides, there are number of plagioclases are most abundant and often conglomerates, of interformational and accompanied by augite (Fig.2C). The clinopyroxenes intrafoemational types within Shillong basin. are augite and zoning is observed from rim to cores. Cretaceous and Tertiary rocks are extensively Olivine grains when alter to augite, form rims around exposed in southern part of the plateau. Thus the olivines (Fig. 2D). Extensive development of with plateau witnessed bsic, acidic and The occurrence variable amounts interstitial magnetite is seen and distribution of the metadolerites of the Shillong (Fig.2E). Quartz and biotite occurs in minor amount. basin are shown in Figure 1. Clusters of plagioclase form relict Petrography glomeroporphyritic texture. Granoblastic texture is seen in the massive types (Fig.2F). Magnetite, Metadolerite are light green to greenish black in ilmenite, apatite and rutile are present as accessories. colour and are coarse to medium grained rocks. They Presenece of relict ophitic to subophitic texture along are essentially composed of augite, plagioclase, with skeletal habit of pyroxene are seen which opaques and rarely olivine in addition to secondary indicate their basic igneous parentage hornblende, actinolite, chlorite, epidote and biotite. The field association and their megascopic

Asian Journal of Multidisciplinary Studies, 3(2) February, 2015 39 Geochemistry of Mesoproterozoic metadolerite dykes and sills of Shillong basin, Meghalaya, NE India.

Geochemistry After careful petrographic studies, 15 representative samples of dykes nd sills were analysed for major, trace and rare earth elements. Whole rock major oxides were analysed by automated sequential spectrometer X- ray fluorescence (XRF) and trace and REE element analyses were done by ICP-MS technique at NGRI Geochemical Laburatory, Hyderabad, following the procedures described by Balaram (2005). The geochemical data of the samples are presented in Table 1. The geochemical data reveals variation in their geochemical characters and indicate that the mafic magma source has differentiated at mantle depths. All metadolerites show SiO2 (53.07-58.92 wt %), total iron as Fe2O3 varies between 10 -17 wt%. The rocks are relatively high in TiO2 content (0.8 to 3.54 wt %) and such high content may be due to crustal contamination. Alumina exhibits a wide variation ranging from 11.22 to 15.86 wt% and the variation is mainly attributed to the fractionation. Iron and magnesia show variation from 10.99 to 16.74wt% and 2.40 to 5.97 wt%, respectively. The moderate Mg # 23-47 suggests that this variation is not only controlled by the fractionation of ferromagnesian silicates but also by iron oxides. The rocks have moderate calcium content of 3.23 to 7.71 wt% and the Na2O (0.83 to2.07 wt %) appears to be low. K2O content varies from 0.15 to 1.96 wt%. Several variation diagrams were used to know the classification of geochemical characteristics and their tectonic emplacement. SiO2 when plotted against TiO2 (wt%) it is observed that SiO2 decreases with increasing TiO2 and negative correlation is indicative of crystal fractionation. Similar negative correlation is shown by Fe2O3, CaO, MgO against SiO2while positive correlation is shown by Al2O3, MnO, Na2O, P2O5 (Fig. 10a). In the total alkali vs SiO2 (TAS diagram after Cox et al. 1979;, metadolerites are plotted in the fields of basalt and basaltic andesite excepting three sample confined to andesite and all plots are characteristically fall in the field of sub alkaline (Fig. 3). All the metadolerites show enrichment of iron during their evolution and showing tholeiitic trend (Fig. 4). Binary variation Nb/Y- Zr/Ti plot shows sub alkaline basaltic composition in the Pearce (1996) diagram (Fig. 5). The tholeiitic character is obviously displayed by plots of FeO(t) /MgO vs SiO2 in the Miyashiro‟s (1974) diagram (Fig. 6). It is supported by another ternary plot of Jensen (1976) MgO-Al2O3- FeO+TiO2 which shows that all the samples have high iron rich tholeiitic characteristics (Fig. 7). Plots of Zr / TiO2 vs Nb / Y for mafic dykes and sills also categorically fall in the sub alkaline field. All the dolerites show quartz normative (q= 19.4-33) and are tholeiitic in composition. Many mafic bodies show corundum in their norms (0.4- 6.0%). SiO2 when plotted against TiO2 (wt %) it is observed that SiO2 decreases with the increased TiO2 and such negative anomaly is another subscribing evidence of crystal fractionation. Ternary plots of TiO2 – P2O5 – K2O and FeO – MgO – Al2O3 clearly indicate that the mafic dykes and sills are of continental tholeitic type (Figs. 8, 9). The LILE (large ion lithophile elements) such as Ba (110- 540 ppm), Rb (5-109 ppm), Sr (122- 486 ppm) show higher concentration. Tectonic discriminatory ternary diagrams like FeO* - MgO – Al2O3 and TiO2 – K2O – P2O5 are self explanatory that they belong to continental setting. Tectonic diagrams are used only as part of a framework that is also based on other evidences. Different major oxides and trace elements plot against SiO2 show systematic variations and supports fractional crystallization from the source magma (Fig. 11a, b). The HFSE (High Field Strength Elements) such as Sc (25- 58 ppm), Y (24-83 except one sample 252 ppm), Th (2-10 ppm), U (up to 3 ppm), Pb (14- 53 ppm), Zr (25- 287 ppm), Hf (1- 27 ppm), Nb 4-30 ppm) and Ta are found to be moderate in these metadolerite dykes and sills. Zr / Nb ratio ranges from 2.2-15, Zr / Hf ratio of 25- 71, Zr / Nb ratios of 2.1- 15 indicates strongly enriched character. Fig. 9 REE in the metadolerites varies from 56 to 281 ppm, and show

Asian Journal of Multidisciplinary Studies, 3(2) February, 2015 40 Geochemistry of Mesoproterozoic metadolerite dykes and sills of Shillong basin, Meghalaya, NE India.

fractionation trends (La/Yb)N= 4.1- 9.8. Most of the samples enriched in LREE (LaN/SmN =1.8-2.5) and relatively similar enriched HREE (GdN/YbN = 1.9-3.3) trends. The Europium anomaly in these samples is nearer to one indicating that the plagioclase feldspar has not played significant role. Eu/Eu* varies between 0.8-1.1 and our earlier reported results are also support similar geochemical characters (Mallikharjuna Rao et al., 2009). La/SmN vary between 1.56 to 2.29. The REE pattern is relatively flat and coherent and negative minor Eu anomaly is observed (Fig. 10a). La/Nb, Th/La, Hf/Sm ratios are useful indicators of crustal contamination and those ratio ranges from La/Nb 1.3 to 3.6, Hf / Sm=0.15-2.16, Th/La range from 0.29 to 0.15. Th/La (upto 0.33) indicates effect of crustal contamination. In multielemental spidergram negative troughs of Sr and Eu also suggest the plagioclase fractionation. In both the spidergram moderate negative peaks is indicated by Nb, Sr, Zr, P, Ta and Ti suggesting that crustal contamination affected the samples chemistry and often strongly positive Pb peaks (Fig.10a, b) are the typical of continental crust. All the diagrams referred to above suggest that the mafic dykes and sills are the products of fractional crystallization, partly contaminated and mantle derived partial melting products. Chemical composition show high silica, high alumina nature, and quartz hypersthenes normative which signify that the rocks are quartz tholeites (Thompson, 1984). SiO2-Zr/TiO2 classification diagram (Winchester and Floyed, 1977) shows sub alkaline character (figure not shown). (La/Lu)N ratio is more than 5, which indicates that the studied samples are genetically related to each other. These mafic rock samples exhibit REE patterns and do not show significant Eu anomaly. It is important to note that HFSE and REE pattern do not show

any significant contamination signatures because contaminated samples should exhibit inconsistent pattern. Samples contaminated by crust should exhibit steeper slope for LREE than HREE as crustal material are enriched in LREE. In this case (La/Eu)N should be higher than (Eu/ Lu)N but it is not. Average Nb / La is 0.5 which is close to Primordial mantle (1.01) (McDonough, et al., 1992), whereas it is 0.23 for Lower crust (Weaver and Tarney, 1984). High Ba / Ta ratios observed suggest involvement of crust. Ray et al. (2013) have noted two types of metadolerite-foliated and massive-and based on isotopic signatures, it was opined that partial melting of a subduction modified, metasomatized, enriched lithospheric mantle wedge of spinel-peridotite is the parent magma source for metadolerites. It is seen that the studied samples are probably derivedfrom a magma contaminated with a minor amount of crustal material, but it requires further chemical data, particularly isotopic data are necessary to establish this. Discussion Stratigraphically, the BGG of amphibolite – granulite transitional facies is regarded as the basement of Palaeoproterozoic age above which, Shillong Group of rocks of Mesoproterozoic age acts as cover rock in the Shillong basin. Two sets of mafic dykes and sills are found to occur both in the basin as well as underlying basement. Partially metamorphosed mafic dykes and sills of Mesoproterozoic age is confined only within the Shillong basin and not in the basement gneisses. In the basement gneiss, unmetamorphosed basic dykes and sills of Cretaceous age (both dolerite and basalt) are seen in the western part of the Shillong plateau (East and West districts of Meghalya) and they are equivalent to Sylhet trap. The geochemical and plaeomagnetic characteristics of the two sets are distinctly different. The metamorphic events in the Shillong plateau are wide spread between 1500-1600 Ma which is the tentative emplacement period of the older group of Mesoproterozoic metadolerite dykes and sills. The plot of Y/Nb and Zr/Nb to show the relationship between Plume related MORB and normal MORB for the metadolerite, the data sets are plotted towards Zr/Nb side of the mixing line between Y/Nb- Zr/Nb which indicate that mantle plume or hot spot may be the supplying source of influx (Mallikharjuna Rao et.al., 2009). The Khasi greenstone dykes and basalts that show continental flood basaltic nature and low Mg# suggest that they were not equlibrated with the mantle rock mineralogies. From the chondrite normalized REE pattern, an overall poor fractionation of REE with no significant Eu anomaly is observed which probably indicate that the rocks have been derived from a melt undergoing shallow level fractionation. Nb/Y vs Rb/Y plot (Fig. 12a) clearly define two groups of samples present, in one group plots are closely spaced andin another group wide spread scatter field for samples. The Ba/Nb vs Sm/Nd ratio are plotted

Asian Journal of Multidisciplinary Studies, 3(2) February, 2015 41 Geochemistry of Mesoproterozoic metadolerite dykes and sills of Shillong basin, Meghalaya, NE India.

and these ratios also define two similar groups. Most of the plots of metadolerite dykes and sills are close to average lower crust and average Indian amphibolites (Fig. 12b).

Fig. 11a

Asian Journal of Multidisciplinary Studies, 3(2) February, 2015 42 Geochemistry of Mesoproterozoic metadolerite dykes and sills of Shillong basin, Meghalaya, NE India.

Fig. 11b

Asian Journal of Multidisciplinary Studies, 3(2) February, 2015 43 Geochemistry of Mesoproterozoic metadolerite dykes and sills of Shillong basin, Meghalaya, NE India.

Conclusions suggest to the fact that the singularly defined Intracratonic Shillong Basin of NE India reaches its The petrological and geochemical signatures of the highest intensity during Neoproterozoic period with metadolerites of the Shillong basin of Meghalaya the influx of felsic magmatism in the form of lead to the following conclusion. The metadoleites various granite plutons. Plots suggest that the mafic of the Shillong plateau are restricted only in the dykes are genetically related to WPB (Within Plate intracratonic Shillong basin and intruded into the Basalt) and such type of continental basalt were metasedimentary metavolcanic sequences of emplaced in a rift related tectonic setting. Shillong Group metamorphosed under of green schist – amphibolite transitional facies . A large These mafic metadolerite dykes do not numbers of mafic dykes and sills of Cretaceous age constitute a greenstone belt of the Shillong plateau are present in the basement gneisses in the western although some speculations are made by few Garo hills of the plateau as well as in the southern workers. They are significantly different from border of the basin as Sylhet trap. Since the MORB or arc trench basalt collisional mechanism metadolerite dykes and sills confined to is not reflected by geochemical signatures. Proterozoic Shillong basin and are older than the Continued rifting within the Shillong plateau in a other Cretaceous group of dolerite dykes and sills post – Precambrian time, led to the formation of emplaced only into the basement, they are not subsequent deposition of younger sediments during correlatable under one tectonic setting. Mesozoic time. Such extensional tectonism assists for emplacement of mafic dykes and sills in The dolerites have low SiO (53.0 – 2 Shillong Basin. Hussain et al. (2004) also 58.9%) suggesting derivation from a ultramafic advocated similar type of setting from central mantle source rather than from crust or partial Indian shield. However, more work is due to melting of crustal rocks. Plots within calcalkaline elucidate two sets of emplacement of mafic bodies and continental basaltic fields probably indicate separated in time and space and detailed that the low grade metamorphic effect resulting palaeomagnetic analysis on this aspect is in the subsequent alteration might have not influenced process. much in the composition of the major oxides The most contaminated samples tend to be the least Acknowledgements differentiated. Enriched LREE, LILE and similar The authors would like to acknowledge the concentrations of HREE distinguished by negative Department of Geological Sciences, Gauhati Eu, Nb, P, Ta in mantle normalised spidergrams University, Guwahati, Assam for providing suggests that they are derived by relatively low facilities to carry out this work. The authors thank degree partial melting. The absence of carbonate the Director, CSIR-NGRI for his keen interest in rock in the Shillong basin probably indicate this study. The authors also thankful to the Ministry unstable depositional environment during of Earth Sciences (DST), Government of India for Mesoproterozoic time. The geochemical signature providing financial assistance in the form of the as well as low grade metamorphism and project (ESS/16/249(5)/2005). polydeformation history of the Shillong basin may

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